The yogg stuff were echos. Echos are not alive. Blizzard have stated time and again that we killed C'thun and Yogg. No one doubts that Y'Saarj was killed and he still is able to communicate (so was the skull of Gul'dan and no one doubt MU Gul'dan was killed too.). Yogg confirms his own death for crying out loud "The shadow of my corpse will choke this land for all eternity." Can't be a corpse if you're not dead.
The nightmare now is run by N'zoth. Xalatath states he's the only one left to be the ultimate victor.
This whole "Old Gods don't die." comes from ramblings of their crazed followers. The worst people to take for objective comments about them.